Recently, portable electronic devices such as a mobile phone, a notebook personal computer, a personal digital assistant (PDA), a video camera, and a digital camera are widely spread. With further requirements in size reduction and weight reduction of such electronic devices, the requirements in size reduction, weight reduction, thickness reduction, and increase of capacity of a battery as a driving power supply are rising, and investigations relating to these problems are actively proceeding. A lithium secondary battery has high voltage and a favorable energy density. For this reason, it has been widely used as a power supply of the portable electronic devices. However, with the requirement of further small-sized and weight-reduced battery along with the development of small-sized and weight-reduced display industries, further improved battery characteristics such as high drive voltage, prolonged life and high energy density as compared with a conventional lithium secondary battery are required. Furthermore, recently, the development of a medium-sized or large-sized lithium secondary battery for automobile use or for industries is proceeding, and expectation is placed on the development in the improvement of high capacity and high output. Therefore, to satisfy those requirements, efforts for improving the performance of various constituent elements of the lithium battery has been continued.
Characteristics of a battery are greatly influenced by an electrode, an electrolyte and other battery materials used. Particularly, in the case of an electrode, the characteristics are determined by an electrode active material, a current collector and a binder imparting adhesive force therebetween. For example, an amount and kind of the active material used determine an amount of lithium ions that can be bonded to the active material. Therefore, a higher capacity battery can be obtained as the amount of the active material is large and the active material having larger inherent capacity is used. Furthermore, in the case where the binder has an excellent adhesive force between the active materials and between the active material and the current collector, electrons and lithium ions smoothly transfer inside of the electrode, and internal resistance of the electrode is decreased. As a result, highly efficient charge and discharge can be realized.
In the case of a high capacity battery, a composite electrode such as carbon and graphite or carbon and silicon is required as an anode active material, and volume expansion and contraction of the active material greatly occur during charging and discharging. Therefore, the binder must have excellent elasticity in addition to excellent adhesive force, and must maintain the inherent adhesive force and restoring force despite that the electrode volume repeatedly undergoes considerable expansion and contraction.
As a binder for obtaining such the electrode, known is one containing a fluorine resin such as polytetrafluoroethylene or polyvinylidene fluoride, dissolved in an organic solvent. However, the fluorine resin does not have sufficiently high adhesiveness to a metal constituting the current collector, and additionally, does not have sufficiently high flexibility. Therefore, particularly, in the case of producing a wound-type battery, there are problems that cracks are generated in an electrode layer obtained and peeling occurs between the electrode layer obtained and a current collector. To maintain sufficient adhesive force, the used amount of the resin must be increased, and therefore size reduction has its limit. Furthermore, the resin is used as a mixture with an organic solvent, and therefore there is a disadvantage that the production becomes complicated.
On the other hand, a binder containing a styrene-butadiene latex (SBR) is known as one having high adhesiveness to a metal constituting a current collector and capable of forming an electrode layer having high flexibility (Patent Documents 1, 2 and 3). However, it has excellent elastic property, but adhesive force is weak, the structure of an electrode cannot be maintained with repetition of charging and discharging, and it cannot be said that a life of the battery is sufficient.
In recent years, in view of the demand for enhancement of the battery capacity, there is a tendency that the content of a binder component as a material constituting the electrode layer is decreased, and the electrode layer is subjected to press molding in the production process of the electrode. In the electrode layer that has a small content of the binder component, however, the electrode layer is liable to be released from the collector during the press molding. Therefore, problems are pointed out not only that contamination of the press molding machine with the electrode substance occurs but also that the electrode is installed in a battery in such a state that an electrode layer is partially released off and thus the reliability of the battery performance is deteriorated. Since such problems may become conspicuous when a polymer having a low glass transition temperature and high tackiness is used as the binder component, those may be suppressed by using a latex of which a polymer has a high glass transition temperature, for example, equal to or higher than room temperature. However, in the case of using a binder of which a polymer has a high glass transition temperature, the resulting electrode layer has low flexibility and thus is liable to suffer cracking, so that there arises problems that the capacity retention rate of the battery is deteriorated and sufficient charge and discharge cycle characteristics are not obtained.